from __future__ import division
from __future__ import unicode_literals

import torch


class ExponentialMovingAverage:
  """
  Maintains (exponential) moving average of a set of parameters.
  """

  def __init__(self, parameters, decay, use_num_updates=True):
    """
    Args:
      parameters: Iterable of `torch.nn.Parameter`; usually the result of
        `model.parameters()`.
      decay: The exponential decay.
      use_num_updates: Whether to use number of updates when computing
        averages.
    """
    if decay < 0.0 or decay > 1.0:
      raise ValueError('Decay must be between 0 and 1')
    self.decay = decay
    self.num_updates = 0 if use_num_updates else None
    self.shadow_params = [p.clone().detach()
                          for p in parameters if p.requires_grad]
    self.collected_params = []

  def update(self, parameters):
    """
    Update currently maintained parameters.

    Call this every time the parameters are updated, such as the result of
    the `optimizer.step()` call.

    Args:
      parameters: Iterable of `torch.nn.Parameter`; usually the same set of
        parameters used to initialize this object.
    """
    decay = self.decay
    if self.num_updates is not None:
      self.num_updates += 1
      decay = min(decay, (1 + self.num_updates) / (10 + self.num_updates))
    one_minus_decay = 1.0 - decay
    with torch.no_grad():
      parameters = [p for p in parameters if p.requires_grad]
      for s_param, param in zip(self.shadow_params, parameters):
        s_param.sub_(one_minus_decay * (s_param - param))

  def copy_to(self, parameters):
    """
    Copy current parameters into given collection of parameters.

    Args:
      parameters: Iterable of `torch.nn.Parameter`; the parameters to be
        updated with the stored moving averages.
    """
    parameters = [p for p in parameters if p.requires_grad]
    for s_param, param in zip(self.shadow_params, parameters):
      if param.requires_grad:
        param.data.copy_(s_param.data)

  def store(self, parameters):
    """
    Save the current parameters for restoring later.

    Args:
      parameters: Iterable of `torch.nn.Parameter`; the parameters to be
        temporarily stored.
    """
    self.collected_params = [param.clone() for param in parameters]

  def restore(self, parameters):
    """
    Restore the parameters stored with the `store` method.
    Useful to validate the model with EMA parameters without affecting the
    original optimization process. Store the parameters before the
    `copy_to` method. After validation (or model saving), use this to
    restore the former parameters.

    Args:
      parameters: Iterable of `torch.nn.Parameter`; the parameters to be
        updated with the stored parameters.
    """
    for c_param, param in zip(self.collected_params, parameters):
      param.data.copy_(c_param.data)

  def state_dict(self):
    return dict(decay=self.decay, num_updates=self.num_updates,
                shadow_params=self.shadow_params)

  def load_state_dict(self, state_dict):
    self.decay = state_dict['decay']
    self.num_updates = state_dict['num_updates']
    self.shadow_params = state_dict['shadow_params']